The present invention relates to a system for controlling the supercharging pressure of an internal combustion engine with a turbocharger.
An internal combustion engine for an automobile is provided with a turbocharger driven by an exhaust gas turbine having a small capacity, so that the turbocharger may operate at a low engine speed. In such an engine, supercharging pressure excessively increases with increase of engine speed, which may cause breakdown of the engine at high engine speed. In order to avoid such a damage, for example, a system has been proposed by Japanese Patent Laid-Open Specification No. 53-76213. The system comprises a bypass provided in the exhaust-gas turbine, a waste gate valve in the bypass, and an actuator for operating the waste gate valve. The actuator comprises a diaphragm operatively connected to the waste gate valve, an atmosphere chamber and a supercharging pressure chamber partitioned by the diaphragm. The diaphragm is deflected by the difference between the atmospheric pressure and the supercharging pressure when the supercharging pressure increases. Thus, the actuator operates to open the waste gate valve to discharge the exhaust gases through the bypass when the engine speed is higher than a predetermined speed. Accordingly, the increase of supercharging pressure is prevented
However, since the atmospheric pressure is low at high altitude, the actuator operates at a low supercharging pressure, so that the supercharging pressure is lower than at low altitude in spite of the same engine speed. Therefore, an increase of the engine output by the turbocharger cannot be obtained at high altitude.